A lithium ion battery has been used practically as a battery having high energy density. As an active material having a high energy density as great as lithium, attention is being paid to calcium and magnesium.
However, there are few calcium salts and magnesium salts soluble in an organic solvent. Regarding calcium salts, only calcium perchlorate has been studied (J. Electrochem. Soc. Vol. 138, pp. 3356-3545 (1991), D. Surbach, R. Skaletsky and Y. Gofer, “The Electrochemical Behavior of Calcium Electrodes in a Few Organic Electrolytes”).
A perchlorate is the most stable among chlorates (oxygen acid chloride). Some perchlorates, however, explode by high heat or shock. There also is a risk of an explosion when the perchlorate is heated or is ground in the presence of a combustible material.
Calcium perchlorate is one of the perchlorates having a characteristic as described above. Therefore, there is a great obstacle to using it as an electrolyte for a battery because of the risk involved. A calcium salt which is capable of being handled safely and of being dissolved in an organic solvent has been looked for use in an electrolyte for a calcium ion battery.